1. Field of Invention
The present invention relates to seismic exploration. More particularly, the invention concerns a method for attenuating flexural ice waves and random noise during seismic exploration in arctic regions by using signals from hydrophones and geophones.
2. Description of Related Art
Due to the increasing-cost of importing petroleum products such as crude oil, more and more attention has been given to locating new petroleum resources in remote regions such as the arctic. And, like seismic exploration in non-arctic regions, seismic exploration in the arctic regions is usually performed using surface energy sources, rather than "shothole dynamite" or other older methods. Surface seismic energy sources are favored for economic as well as environmental reasons.
An example of a surface energy source is the Vibroseis.RTM. system (Vibroseis is a registered trademark of Conoco). With the Vibroseis system, a movable reaction mass is utilized to direct low frequency vibration signals through the earth. Typically, a specially-equipped truck contains a housing that supports the reaction mass. During operation, a base plate is coupled to solid ground, such as the earth's surface or a layer of ice that covers the earth's surface. The reaction mass is connected to a two-sided piston and the base plate. High pressure hydraulic fluid reciprocates within the housing surrounding the piston, causing the reaction mass to vibrate up and down with respect to the ground. The vibration of the reaction mass generates low frequency signals that penetrate the earth through the base plate.
As with other methods using surface sources, the seismic signal is reflected with varying angles and intensities by various matter within the earth. One or more seismic detectors receive the reflected signals at the earth's surface, a memory device stores the received signals, and a computer is used to refine and interpret the stored signals.
In the Vibroseis system, the reaction mass is vibrated to generate a constant amplitude seismic signal having a frequency that varies linearly or nonlinearly with time. The received signal, when cross-correlated with the signal generated by the vibrating energy source, is similar to reflected signals originating from explosive sources.
When using a surface energy source to conduct seismic exploration in arctic regions, the seismic data may contain noise in some situations. Specifically, when a layer of water exists between a layer of floating ice and the earth, the ice is permitted to flex in response to vibrational seismic signals created by seismic energy sources. In some cases, the ice may even move as much as 2-3 centimeters near the energy sources. This flexion of the floating ice sheet generates high amplitude dispersive noise waves (called "flexural ice waves" or "flex waves", for short). The flexural ice waves efficiently propagate from the seismic sources through the ice the receivers, and severely mask the simultaneously arriving, weaker seismic signals reflected by the earth. In some cases, flexural ice waves on the order of 80 decibels (dB) have been recorded. In other words, seismic data signals may be a factor of 10,000 times weaker than overlying flexural ice waves.
Floating ice sheets are often associated with seasonal ice, i.e., ice that alternately forms and disappears with changes of seasons. However, when an ice sheet is in direct contact with the land beneath it, flexural ice waves are negligible, since the contact between the ice and the earth limits ice flexion to small amounts.
When the Vibroseis system is utilized in conjunction with a floating ice sheet, one or more geophones are affixed to the surface of the ice. The flexural ice waves interfere with the data received by the geophones attached to the ice, since the flexural ice waves propagate horizontally through the ice sheet. One method to eliminate the noise created by flexural ice waves is discussed in "Surface Sources on Floating Ice: The Flexural Ice Wave," by Lansley, Eibert, and Nyland, 54th Annual International Meeting of the Society of Exploration Geophysical, Expanded Abstracts, 1984, pp. 828-831. The Lansley method utilizes an "ice saw" to cut an opening through the ice sheet, somewhere between the seismic source and the receivers, and thereby create discontinuities in the ice that help attenuate flexural ice waves.
With the Lansley method, the discontinuities in the ice have been known to provide about 20 dB of flexural ice wave attenuation. However, for some applications, the Lansley approach is not as useful as some might like. In particular, the Lansley method is limited since, even though Lansley cuts an opening in the ice, the flexural ice waves may pass around the opening, introducing substantial interference into signals created by hydrophones and/or geophones used in the system.
Other methods have approached the flexural ice wave problem with various processing algorithms. One example is that of Barton et al, entitled "Flexural Ice Waves On Arctic Data: Application of New Techniques for S/N Enhancement," 56th Annual International Meeting of Society of Exploration Geophysicists, Expanded Abstracts, 1986, Session S8.7. Another example of a processing algorithm is McConnell et al "Dispersive Noise Attenuation," 56th Annual International Meeting of Society of Exploration Geophysicists, Expanded Abstracts, 1986, Session S8.6. Although these methods have achieved some improvement, and may even be satisfactory for some purposes, they are not entirely adequate in some applications.